Small, high performance accelerometers are typically made of silicon. As shown in FIGS. 1 and 2, accelerometers commonly employ a planar construction method. A monolithic silicon substrate is micro-machined to yield a pivot mechanism, usually a proof mass suspended from a stationary frame by flexure hinges. The hinges allow rotation of the proof mass about a hinge axis. Top and bottom cover plates are used as damping surfaces and shock stop restraints. Referring to FIG. 2, these cover plates are then bonded to the mechanism that includes the suspended proof mass to form a completed die stack. The die stack in turn is bonded into a header with appropriate drive electronics attached to form the completed accelerometer.
Unfortunately, these bonding operations directly impair accelerometer performance. Available bonding materials, such as epoxy, glass frit, etc., generally have a coefficient of thermal expansion substantially different from that of the silicon. Because the bonding is usually performed at elevated temperatures, there is a standing internal stress condition between the silicon and the bond joints. The delicate sensing mechanism often becomes warped during this bonding process. Relaxation of the internal stress over time and temperature generates drift and hysteresis that limit accelerometer performance.
Attempts have been made to isolate the basic mechanism from bond joint stress. An illustrative scheme is presented in FIG. 3. This arrangement involves suspending the acceleration mechanism on a frame, forming the cover bond joints on an outer rim and connecting the frame to the outer rim with suspension beams. This and other prior art arrangements have alleviated the problem somewhat, but they fail to address the source of the problem. Also, the isolation feature generally adds the expense of extra silicon and design complications. Similar arguments apply to joints attaching the completed sensor mechanism to the package header in a completed accelerometer.